The present invention is an apparatus to improve the lateral resolution performance of ultrasonic sensor arrays, particularly when such arrays are used to achieve substantial penetration distances into a body or object under investigation, as in medical research, diagnosis and examination.
Ultrasonic imaging sensors for medical applications usually transmit frequencies in 4 to 5 megahertz range. However, because high frequencies are attenuated more rapidly than lower frequencies, the return signal received will have a lower frequency content than the transmitted signal. As shown in FIG. 3, a 5 Mhz input will be downshifted to 2.5 Mhz following a 27.5 cm round trip in the body.
Lateral resolution is a measurement of how closely two items can be located together and still be separated by an imaging sensor. The spacing which can be resolved is a direct function of wavelength divided by sensor aperture. Thus to achieve fine lateral resolution, it is necessary to use a short wavelength, a large aperture, or best of all, both. When the object being imaged is at some distance inside of an attenuating medium, the initial frequency is "downshifted" as it penetrates deeper into the body. Thus, the frequency returned will be lower than that transmitted into the body, and lateral resolution will be lower than expected. Downshifting is observed when, for example, an organ deep within the human body is imaged. The deeper the organ, the less lateral resolution can be achieved. It is extremely important to be able to achieve a high resolution of organs deep within the body.
There is a deeply felt need within this industry for a means of achieving good lateral resolution of targets which are deeply imbedded within an attenuating medium, such as organs within the human body. What actually occurs is that the high frequency portion of the signal is attenuated more rapidly than the lower frequencies, so that the return signal is shifted to much lower frequencies than the signal which was originally transmitted.